GPR Signal Research Articles

A practical approach to correct attenuation effects in GPR data

As the GPR pulse propagates in the geologic medium, it suffers attenuation as the result of absorption and dispersion, besides spherical divergence. High frequency components of the pulse suffer more pronounced effects of attenuation than low frequency components resulting in resolution loss in radargrams. The quality of the GPR images is then strongly dependent on an adequate correction of the attenuation effects. We present a practical approach to approximately correct the attenuation effects based on their influence on the amplitude and phase spectra of GPR signals. The key point of this approach is the fact that in low and moderate losses dielectric media the propagated signal suffers significant changes only in its amplitude spectrum; that is, the phase spectrum of the propagated signal remains practically unaltered for the usual travel time ranges. Based on this fact, we show using real data (both CMP and radargram) that the judicious application of the well known tools of time gain and spectral balancing can efficiently correct the attenuation effects, from the practical point of view. The proposed approach can be applied in heterogeneous media and it does not demand the precise knowledge of the attenuation parameters of the media. This approach should be understood as part of an interpretative data processing, which can be done by using the interpreter's knowledge on the studied area and even during the fieldwork. As an additional benefit, the judicious application of spectral balancing promotes a partial deconvolution of the data without changing its phase. In this way, the spectral balancing acts in a similar way to a zero phase deconvolution. Usually, in GPR data, the resolution increase obtained with spectral balancing is greater than those obtained with spike and predictive deconvolutions.

Exploring Independent Component Analysis for GPR Signal Processing

Independent component analysis (ICA) is a kind of blind signal processing (BSP) or blind source/signal separation (BSS) technique that develop quickly in recent years. It has found many applications in a variety of fields and been successfully used to acoustical signal processing, biomedical signal processing, feature extraction, edge detection and face recognition, etc. In this paper, based on the analysis to the GPR signal characteristics, we have primarily explored ICA for GPR signal processing using FastICA. A primary study on FDTD simulation and experimental data being processed by FastICA has indicated ICA technique in GPR signal processing will be promising. In our approach, the separation of target signal and background signal has been implemented automatically. We also primarily settled the ambiguities of the sign of the separated target signal components. Our primary results have showed it can effectively remove the background in the raw GPR data and keep the fineness of target signal simultaneously. Additionally, our method may be used on-line. We reckon ICA can become a powerful technique in GPR signal processing.

Electromagnetic Inversion of GPR Signals and Subsequent Hydrodynamic Inversion to Estimate Effective Vadose Zone Hydraulic Properties

We combine electromagnetic inversion of ground penetrating radar (GPR) signals with hydrodynamic inverse modeling to identify the effective soil hydraulic properties of a sand in laboratory conditions. Ground penetrating radar provides soil moisture time series that are subsequently used as input in the hydrodynamic inverse procedure. The technique relies on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with an off‐ground monostatic transverse electromagnetic (TEM) horn antenna. Ground penetrating radar signal forward modeling is based on the exact solution of the three‐dimensional Maxwell equations for describing free wave propagation and on linear systems in series and parallel for describing wave propagation in the antenna. Water flow in the sand is described by the one‐dimensional Richards equation using the Mualem–van Genuchten parameterization. Both model inversions are formulated by the classical least‐squares problem and are performed iteratively using advanced global optimization techniques. Compared with time domain reflectometry (TDR), results demonstrated the appropriateness of the GPR integrated approach to measure soil moisture remotely. In particular, the approach was found to be less sensitive to the inherent small‐scale heterogeneities. Hydrodynamic inversion of soil moisture data led to hydraulic parameters agreeing reasonably well with direct measurements. The observed discrepancies were attributed to the different characterization scales and samples. The overall integrated approach offers great promise to map the effective hydraulic properties of the shallow subsurface at a high spatial resolution.

Suppressing GPR Clutter from Randomly Rough Ground Surfaces to Enhance Nonmetallic Mine Detection

This study attempts to quantify the ground penetrating radar rough ground surface clutter by numerical modeling of wave scattering, and establish a strategy to suppress the clutter for given test signals. The goal is to improve the GPR detection statistics for small, buried, low-contrast nonmetallic antipersonnel mines. Using a model of an experimentally measured impulse GPR signal, we simulate the ground surface and buried low-contrast mine target scattered responses. We employ a 2D finite difference time domain (FDTD) method to analyze the pulse shape, delay, and amplitude characteristics of the scattered waves—with and without buried nonmetallic mine targets—as a function of roughness parameters. Five hundred Monte Carlo simulations of various test cases of specified ground root mean square height and correlation length were run to generate statistics for the clutter and target signal variations. In addition, the effectiveness of identifying and removing the ground surface clutter signal for detecting subsurface targets is presented. Results indicate that even with moderate roughness, statistics can be generated to enhance the detection of small, shallow, low-contrast targets.

Field experiments for characterization of GPR antenna and pulse propagation

Productive interpretations of ground penetrating radar surveys require an accurate understanding of electromagnetic wave radiation, propagation, and scattering in geological materials as well as accurate knowledge of the reflection characteristics of various target anomalies embedded in such materials. GPR responses and survey profiles are often interpreted on the basis of theoretical estimates and numerical simulation models of electromagnetic wave propagation in simplified representations of ground materials and by using idealized target contrasts and geometries. Alternatively, field experiments performed under controlled test conditions can also be effective in demostrating GPR system performance capabilities and in providing quantitative measurements in realistic geologic formations. Experimental research at the University of Rome “La Sapienza” and at the Italian National Research Council were initiated to develop a basic understanding of the radiation and scattering characteristics of VHF pulse-mode GPR signals in earth materials and in air with emphasis on antenna ground coupling and target backscatter responses. The results of the experimental measurements conducted in air provided baseline information on the GPR system and target reflections under lossless propagation conditions. Target response measurements at various burial depths provided a systematic data base from which target responses, propagation parameters of the medium, and relevant data processing techniques were evaluated to gain useful insights into their interpretations. Other more advanced experimental tests are planned for the future.

Mars 96 GPR program

Evolution models of the planet Mars show that a significant quantity of water may reside beneath the surface as ground ice. This permafrost, which at mid-latitudes could be 1 to 2 km deep, forms a better penetrating medium for radio waves than warmer dry ground. A GPR system is planned to be integrated inside the surface-drag guiderope of the Mars 96 balloon mission to detect the permafrost, estimate its thickness, and to evaluate the water quantity. We have developed a GPR system for operation within an experimental guiderope. This implies techniques and technologies adapted for deep sounding, compatible with the Mars 96 mission weight and volume constraints, cold temperatures, and shock resistance. This also provides a basis for electromagnetic compatibility, antenna, propagation studies, and simulations. In this paper, we present an impulse radar system using repetitive-trace coherent integration to improve the signal-to-noise ratio, and a variable gain receiver to increase the dynamic range as the echo depth increases. The outermost shell of the guiderope assembly forms a loaded dipole antenna. Preliminary experiments were carried out on a glacier and on a sand dune to obtain representative experimental GPR profiles. We present the processing and the interpretation of these profiles in combination with modelling of the medium and numerical simulation of the GPR signal propagation. Finally, using possible geological models of the Martian subsurface, we estimate the radar received power and discuss the capability of the GPR for imaging the Martian permafrost.